The present invention relates to structure capable of compensating for the tendency of a beam to become deflected as a result of loads which act on the beam and as a result of temperature differentials existing in the beam.
For example, in the headbox of a paper machine, or in an equivalent construction, there is at the front wall of the headbox an elongated beam supported at no less than two points or locations, most appropriately at the so-called Besselian points or in the region thereof, so as to be carried in this way by the frame beam or structure of the headbox.
It is well known that the front wall beam of a headbox of a paper machine forms the component thereof which carries the upper lip structure defining the upper edge of the slice of the headbox. As a result deformation of such a front wall beam and deflection thereof will influence the geometry of the lip slice and therefore the transverse profile or cross section of the stream of pulp stock which flows through the slice onto the wire.
In accordance with known practice in the prior art, in order to attempt to reduce deflection and deformation of such a front wall beam, such a beam is provided with an exceedingly steady structure while at the same time it is supported at the so-called Besselian points according to which, as is well known, the deflection caused by the weight of the beam itself is maintained at a minimum. However, deflection and deformation of such a front wall beam are caused not only by the weight of the beam itself but also by the pressure load imposed by the stock suspension flowing in the headbox as well as by temperature differentials which necessarily exist between the exterior and the interior of the beam. This temperature differential is highest when the paper machine is being started. At such a time the outer surface of the front wall beam of the headbox has the same temperature as the room temperature prevailing in the paper machine gallery, this temperature being on the order of approximately 20.degree. C. At the same time, however, the inner surface of the beam has a temperature which is the same as that of the pulp stock in the headbox, this latter temperature being on the order of 50.degree. C. Thus, there is a temperature differential on the order of 20.degree.-30.degree. C. This temperature differential will cause a considerable deflection of the front wall beam, and this latter deflection will remain for an interval of from 15-30 minutes up to a few hours after the paper machine has started operating, until a stable condition is reached with respect to the temperature-differential and deflection. The pressure load from the stock suspension, on the other hand, remains substantially constant and starts to exert its influence on the beam immediately after the machine has been started.
Because of the above conditions even though the beam is made of a sturdy construction and even though it is supported in the best possible way with respect to minimizing deflection into the weight of the beam, nevertheless particularly during the starting-up interval there is undesirable deflection of the beam creating an undesirable cross section for the slice of the headbox.